The present invention relates generally to the shielding of electronic devices from electromagnetic interference and in particular, to a gasket which minimizes the amount of conductive material necessary to provide an effective electromagnetic seal.
In the area of electromagnetic interference shielding, particularly for the computer industry, a problem has arisen. With reference to FIG. 1, on the backside of a typical personal computer or business small workstation 2, is a relatively large opening covered with panel 4 to accept necessary connections, e.g., mouse, keyboard, printer, telephone line, etc. This panel presents extraordinary problems to the electromagnetic interference shielding engineer due to the variety of openings or seams which can radiate energy from within the device to the environment.
FIG. 2 is an overhead cut-away view of the back of a typical computer. An intermediate plate 7, usually light-gauge stainless steel, is sandwiched between the frame of the case 14 and the connectors 10 which interface with the motherboard 17. Due to irregularities in the frame 14, the stainless plate 7, and the connectors 10 themselves, a number of gaps and slot openings are created which tend to leak electromagnetic energy into the room. In the last few years, with the frequencies of the emissions becoming greater and greater, the need to shield these gaps and slot openings has increased. The conventional nickname which describes this area is the input/output plane, hereafter referred to as the xe2x80x9cI/O backplane.xe2x80x9d
The most popular fix at the current time is to seal this area with a gasket of the type illustrated in FIGS. 3-5. The gasket is comprised of thermoplastic foam 18 which has been wrapped with a conductive fabric 21. The gasket is in a strip form with a width approximately equivalent to the width of the I/O backplane opening. The gasket thickness is approximately xe2x85x9 inch, or enough to make contact between the connectors and the intermediate stainless steel plate. The holes 24 in the fabric strip shown in FIG. 4 are die-cut away so as to allow the connectors to be accessible to the end user. The termination to ground is made by the fabric 21 contacting the connector base, and grounding to the intermediate plate 7. The intermediate plate 7 must make contact to the frame 14, and this is usually achieved by several stainless steel fingers or divets 15 which protrude off the edges of the intermediate plate 7 and rub against the inner wall of the frame 14.
The problems or shortcomings of the current approach to the problem are as follows:
1. The ground between the intermediate plate and the gasket allows energy to travel along the interface and can reach the end of the gasket which has an open slot. Energy can leak at either end and result in a concentration of energy, or an antennae effect. Presumably, this can also occur at the die-cut holes, since the action of making the hole exposes the non-conductive foam core. That is, the die-cut hole is another antennae.
2. The construction of the fabric over thermoplastic foam is only fair in its physical performance attributes. The foam core is prone to compression set, particularly in the case of urethane foam. After as little as thirty days in compression, the urethane can take up to a 50% set at room temperature. Furthermore, the fabric imparts a stiffness to the strip which increases the deflection force needed to compress the gasket.
3. Multiple manufacturing steps must be performed. The fabric is woven, and subsequently plated with conductive metal. The fabric must then be slit to the proper width. The foam portion of the gasket is made by many manufacturers in a separate step. Usually, the fabric is wrapped around the foam separately with an adhesive. Subsequently, the gasket has an outer pressure-sensitive adhesive applied which provides the end user a means to fasten the gasket to the intermediate plate.
4. The intermediate plate is relied upon to provide attachment and stability to the frame. Unfortunately, the intermediate plate itself is somewhat flimsy and not always the best support to handle the compressive load of the gasket.
In accordance with the present invention, thin strips, or bars, of a conductive elastomeric material are interposed in a primary gasket material to provide a conductive path between conductive surfaces on either side of the gasket where the conductive bars are exposed.
In accordance with a further aspect of the invention, the gasket is generally planar and portions of the primary gasket material are cut out to permit connectors to pass through as is desirable in a gasket for an I/O backplane. The cut out portions expose the conductive bars to the connector thereby providing a conductive path to ground.
It is therefore an object of the present invention to provide a flexible conductive gasket for providing effective electromagnetic interference shielding while minimizing the amount of expensive conductive material used.
It is a further object to provide a flexible conductive gasket which solves the above-mentioned problems with the prior art.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.